As will be described below, in some detail, in order to show that this invention is not a trivial conception, the health problem arises from a kind of cyclotron resonance. This is set up in the blood stream and in our body fluids, under the influence of weak electromagnetic fields at power supply frequency, owing to the combined action of the Earth's magnetic field of some 50 microtesla and, in the applicant's opinion, the thermally activated motion of hydrodium and hydroxyl ions, which are predominant in water but also present in our blood cells.
These ions have molecular masses which combine collectively in reacting to oppose the geomagnetic field and it just so happens that the ratio of the respective reactions of these ions can adjust to screen the influence of the geomagnetic field intensity in a way which results in a prefectly tuned resonant condition, provided that power frequency is within the range 40 to 67 Hz in a 50 microtesla geomagnetic field. The hydrodium ion has a molecular mass of 19 atomic mass units, corresponding to a 40 to 60 Hz cyclotron resonance, whereas the hydroxyl ion has a molecular mass of 17 atomic mass units, implying a 45 to 67 Hz cyclotron resonance. The range in each case is caused by the influence of the field screening effects of the other ion form and the range overlap represents the critical frequency range which can cause the build-up of ion activity leading to the health risk.
Essentially, therefore, the background to this invention concerns the need in U.S.A. to avoid proximity with electric currents or voltage fields at the 60 Hz power frequency. Similarly, in U.K, for example, the risks are the same notwithstanding their 50 Hz power frequency, because both frequencies lie in the danger range.
The electric blanket typifies a situation where the danger elements combine. Firstly, there is the close proximity of the human body to the heating surface. Secondly, there are significant electric currents adequate to sustain the power needed for adequate heating. Thirdly, the conductors are well spaced to keep the forward and reverse flows or voltage polarities apart, so avoiding risk of fire or flashover, but meaning a non-cancellation of the stray electromagnetic fields.
The remedy on the basis of cyclotron resonance is to use dc current to power an electric blanket, but that is something that is not an obvious practical consideration, because it adds circuit components to convert ac into dc and that is not seen as necessary since both are equally effective in producing heat. One may assume, however, that dc has been used to power an electric blanket, where electric storage batteries are relied upon or where domestic power is dc in form.
The invention is solely concerned with heating of blankets and surface heating generally, using apparatus powered by a single-phase ac supply. The heating element has two terminal connections. It is, for example, known to power a surface heating system having two such terminals, relying on a three-phase supply. In this case a three-phase rectifier bridge network is needed to translate the power input into a form that can be connected to the two-terminal heating system. This is disclosed in FIG. 6 of the Orosy and Matlen U.S. patent Ser. No. 3,789,190.
It is obvious to experts in electrical engineering that, given a two-terminal device which is solely intended to be fed by electric current for ohmic resistance heating purposes, the current can be of ac or dc form. The obvious connection is a direct use of unrectified ac if the supply is single-phase, meaning two supply lines. To adapt to a three-phase supply which loads each phase equally, the obvious action is to use a three-phase bridge rectifier configuration as shown in the above referenced U.S. patent, expressly to provide the two supply lines for connection to the two-terminal heating device.
It is not at all obvious to take the unnecessary step of rectifying single-phase ac for such a purpose. Even recognizing that harmful electromagnetic effects do arise from the power frequency excitation of electric blankets, the logical solution would seem to be to use smoothed dc so as to avoid any frequency resonance. The problem with this, apart from the added expense of the circuit components needed to exclude ac current, is that inductive chokes imply other hazards. The switching of power through an inductance can itself give rise to a fire risk which needs addressing by suitable switch design.
Moreover, the research on the health hazard problem posed by electric blankets leaves open in many minds the issue of which frequencies are harmful. It is here that the unpublished research of the applicant has revealed that the hazard range is confined to a frequency range centered on the power frequencies conventionally used in U.S.A. and U.K. The three-phase full-wave bridge-rectified current suggested as the heating power in U.S. Pat. No. 3,789,190 will eliminate the 60 Hz frequency and feed power instead by a mixture of dc and ac components at multiples of 180 Hz. Such a system will achieve for three-phase applications what the applicant is suggesting by this invention for single-phase applications. However, that three-phase advantage is fortuitous, inasmuch as the teachings involved do not suggest the specific health hazard avoidance which is this applicant's objective. For the single-phase application, a necessary condition for most domestic situations, it is necessary to take note of what is here disclosed as the invention by reference to novel structure in rectifying without smoothing the single-phase ac fed into the ohmic resistance of a heating element which has widely spaced compessating current carriers. These have the form of non-convoluted lengths of resistance wire embedded in the fabric of an electric blanket or set in concrete in an underfloor heating system. In contrast with normal electrical flex in which the go and return current paths are intertwined in the convolutions of the wire and so produce cancelling external electromagnetic field effects, the fields set up by two widely spaced conductors carrying currents in opposite directions can augment one another over a range commensurate with that spacing. When lying on an electric blanket or standing on a floor having underfloor heating, parts of the human body are subjected to the full field effects, because the spacings between those conductors are measured in inches.
The prior art disclosures also include circuit arrangements useful in heating electric blankets and containing thyristors or silicon controlled rectifiers to power the system using single-phase ac. Here it must be emphasized that a rectifier which operates under control to interrupt an ac supply as a function of threshold levels of a recurrent voltage waveform will generate ac components having the same frequency as the supply, plus components at integer multiples of that frequency. The 60 Hz or 50 Hz component of a mains power supply will not normally be eliminated by the use of thyristors or silicon controlled rectifiers controlling the power fed to a heating element. It is only if the single-phase voltage supply is full-wave rectified that the alternating current components translate into double frequency components and that there is complete exclusion of the source frequency.
Therefore, disclosures such as that shown in FIG. 3 of Endo, Shinoda and Kimata U.S. Pat. No. 4,658,119, where an electric blanket is supplied through a thyristor, are not relevant in anticipating the invention to be described.